At present, various kinds of display elements are used in practice among which liquid crystalline display elements have such beneficial characteristics as no eyestrain because of their light-receiving property, extremely low consumption of electric power, a thin structure, etc. and are widely utilized for watches, electronic desk computers, personal word processors, etc. However, these display elements utilize nematic liquid crystals and so have a shortcoming that they are slow in response speed. Although various studies have been made to overcome this shortcoming, a satisfactory improvement has not yet been made to overcome this shortcoming.
For the above reasons, studies different from those using nematic liquid crystals are also actively made, one of which uses strongly dielectric liquid crystals.
The ferroelectric smectic liquid crystals were found by R. B. Meyer et al. [J. Physique, 36, L-69(1975)]. As the ferroelectric smectic liquid crystals enable high speed response as compared to the nematic liquid crystals, studies for display elements using the strongly dielectric liquid crystals have now been actively made [N. A. Clark, S. T. Lagerwall Appl. Phys. Lett. 36, 899(1980)].
As the display of this type utilizes ferroelectric smectic liquid crystals, i.e. chiral smectic C phase (referred to hereinafter briefly as SmC* phase) and chiral smectic H phase (referred to hereinafter briefly as SmH* phase), liquid crystalline substances in which these phases exist at around room temperature are desired.
As in the case of nematic liquid crystals, the ferroelectric smectic liquid crystals for display elements are not used singly but are used as a composition comprising plural kinds of ferroelectric smectic liquid crystals.
Generally, suitably selected single compounds are mixed to prepare a composition having the required properties. Among known compounds, however, the number of ferroelectric smectic liquid crystalline compounds utilizable as ingredients for such composition is extremely small. Accordingly, development of new compounds, especially substances having a ferroelectric smectic liquid crystalline phase at about room temperature is desired and strongly demanded. It is an object of the present invention to provide new substances satisfy such demand.
Among the known biphenyl compounds having a similar structure, those having no substituent on their benzene rings are disclosed, for example, in Japanese Laid-open Patent Appln. Nos. Sho. 59-118744, 62-56455, 60-218358, 60-13729, 62-187421, 63-22533, 59-219251 and 60-51147, and those having a halogen atom or atoms on their benzene rings are disclosed, for example, in Japanese Laid-open Patent Appln. Nos. Sho. 61-249953 and 62-255446.
The present invention provides substances having a ferroelectric smectic liquid crystalline phase at about room temperature and being excellent in practical value as compared with these compounds.